From caaa22ced1e3cc9e7c6c79192259a5436f1783e3 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Fri, 3 Aug 2018 22:47:46 -0400
Subject: [PATCH] Entity add to world fixes

1) Chunk Registration might kill an entity, don't add it to the world if it did!

2) By default, entities are added to the world per slice iteration.
This opens risk of the slices being manipulated during chunk add if an
EntityAddToWorldEvent spawns an entity into this chunk.
Fix this by differing entity add to world for all entities at the same time

3) If a duplicate entity is attempted to add to the world of an entity, and
the original entity is dead, overwrite it as the logic does for unloaod queued entities.

diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index 01abe5e376..4502ece4dd 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -2,6 +2,8 @@ package net.minecraft.server;
 
 // Paper start
 import com.destroystokyo.paper.PaperWorldConfig.DuplicateUUIDMode;
+
+import java.util.Arrays;
 import java.util.HashMap;
 import java.util.UUID;
 // Paper end
@@ -957,15 +959,16 @@ public class Chunk implements IChunkAccess {
             // Paper end
 
             // CraftBukkit start
-            List<Entity> toRemove = new LinkedList<>();
-            this.world.a(entityslice.stream().filter((entity) -> {
-                if (this.needsDecoration && !CraftEventFactory.doEntityAddEventCalling(this.world, entity, CreatureSpawnEvent.SpawnReason.CHUNK_GEN)) { // Only call for new chunks
-                    toRemove.add(entity);
-                    return false;
-                }
-                return !(entity instanceof EntityHuman);
-            }));
-            entityslice.removeAll(toRemove);
+            this.world.addChunkEntities(entityslice.stream() // Paper - add all at same time to avoid entities adding to world modifying slice state, skip already added entities (not normal, but can happen)
+                // Paper start - Inline event into stream
+                .filter((entity) -> {
+                    if (!this.needsDecoration) {
+                        return true;
+                    }
+                    return CraftEventFactory.doEntityAddEventCalling(this.world, entity, CreatureSpawnEvent.SpawnReason.CHUNK_GEN);
+                })
+                // Paper end - Inline event into stream
+                .filter((entity) -> !(entity instanceof EntityHuman || entity.valid))); // Paper - add all at same time to avoid entities adding to world modifying slice state, skip already added entities (not normal, but can happen)
             // CraftBukkit end
         }
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index 3acea908c2..e31e366249 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1037,6 +1037,7 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
             }
 
             this.getChunkAt(i, j).a(entity);
+            if (entity.dead) return false; // Paper - don't add dead entities, chunk registration may of killed it
             this.entityList.add(entity);
             this.b(entity);
             return true;
@@ -2434,9 +2435,13 @@ public abstract class World implements IEntityAccess, GeneratorAccess, IIBlockAc
         return j;
     }
 
+    public void addChunkEntities(Stream<Entity> collection) { a(collection); } // Paper - OBFHELPER
     public void a(Stream<Entity> stream) {
         org.spigotmc.AsyncCatcher.catchOp( "entity world add"); // Spigot
         stream.forEach((entity) -> {
+            if (entity == null || entity.dead || entity.valid) { // Paper - prevent adding already added or dead entities
+                return;
+            }
             this.entityList.add(entity);
             this.b(entity);
         });
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index 293818b196..4cda6cee2b 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -967,7 +967,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
             if (this.entitiesByUUID.containsKey(uuid)) {
                 Entity entity1 = (Entity) this.entitiesByUUID.get(uuid);
 
-                if (this.g.contains(entity1)) {
+                if (this.g.contains(entity1) || entity1.dead) { // Paper - if dupe is dead, overwrite
                     this.g.remove(entity1);
                 } else {
                     if (!(entity instanceof EntityHuman)) {
-- 
2.21.0